The generation of frequency modulated continuous wave, namely FMCW signal source, is an important part of the frequency domain sampling reader of the surface acoustic wave (SAW) radio frequency identification (RFID) system. To satisfy the requirements of frequency sweep speed, bandwidth and linearity, the ultra-high frequency (UHF) FMCW signal source is designed by the mixing of direct digital frequency synthesizer (DDS) and phase locked loop (PLL) and combining with IQ modulation. The signal source circuit is actually fabricated. The DDS chip outputs the dual-channel quadrature signals, I and Q, which are transmitted to IQ modulation chip in differential form for up-conversion respectively. The differential and orthogonal characteristics of the DDS output signals are tested; the single frequency signal and sweep frequency signal generated by signal source are tested respectively. Finally, a test system is built to measure the SAW tag, and the test results show the effectiveness of the design of the FMCW signal source.